This application claims the priority of 198 10 464.2, filed Mar. 11, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a crankcase for an internal-combustion engine having crankcase top and bottom sections. Furthermore, the present invention also relates to a process for producing a crankcase for an internal-combustion engine.
In internal-combustion engines whose crankcase top section and crankcase bottom section are made of a light-metal material, such as aluminum or magnesium, problems frequently occur at the bearing points for the crankshaft made of an iron material because the bearing play and the bearing shape change when the temperature changes because of the different coefficients of thermal expansion of the crankcase material and of the crankshaft material. The bearing bore enlarges as the temperature increases, specifically by a much larger factor than the factor of the enlargement of the crankshaft.
Thus, with rising temperatures, there is an increased bearing play between the bearing bore in the conventional crankcase and the crankshaft situated in the bearing bore, which leads to high oil throughputs and to a falling oil pressure, to increasing running noises and to a decreasing load bearing capacity in the lubrication gap of the crankshaft bearing bores. This results in the further disadvantage that, in the case of such crankcases made of light-metal materials, because of the different coefficients of thermal expansion of the crankshaft and of the crankcase, the diameters of the crankcase bearing bore and of the crankcase, as a rule, must be designed such that the crankshaft will jam in its bearing at very low temperatures.
Furthermore, the relatively low stability of the known crankcases is disadvantageous which, particularly with threaded bores for cylinder head bolts and main bearing bolts, results in an overdimensioning of the bores and the associated bolts in order to achieve a sufficient load bearing capacity of the threads.
DE 35 42 137 C2 attempts to solve the problem of the different expansion of the crankcase and of the crankshaft at rising temperatures as well as the connected thermal stressing of the components. Specifically, a crankcase top section made of light metal is equipped with a bearing cap made of gray cast iron which therefore is to serve as a crankcase bottom section. With respect to its thermal expansion behavior, the material of this bearing cap corresponds essentially to that of the crankshaft.
Although this attempted solution results in a slightly lower bearing play as the result of the very different coefficients of thermal expansion of the crankcase top section and of the bearing cap, tensions occur at rising temperatures and a bearing bore cross-section is obtained which deviates considerably from the ideal circular shape. As mentioned above, this, in turn, leads to high oil throughputs and a falling oil pressure.
EP 0 695 866 A1 describes another crankcase for an internal-combustion engine in which a gray cast iron part, which determines the running surface for the piston as well as the bearing points for the crankshaft, is situated in an aluminum casing. In internal-combustion engines with several cylinders, a continuous base block which is made of gray cast iron is therefore obtained to provide, on one hand, a bearing of the crankshaft and, on the other hand, a running surface for each piston.
This just-described known crankcase has the disadvantage of large differences with respect to the coefficients of thermal expansion of the gray cast iron base block and of the aluminum casing. Along the entire length of an internal-combustion engine having several cylinders, when the temperature rises, such expansions and deformations may occur that, as a result, the function of the internal-combustion engine can be considerably impaired. If such a deformation is not possible, the entire crankcase may break.
Another disadvantage of this known crankcase is the fact that the running surface for the piston is made of the same gray cast iron material as the entire base block and therefore no tribological optimum exists with respect to the piston running surface and the piston rings. If, in contrast, the material of the base block is selected according to tribological aspects, stability cannot be optimized.